1. Field of the Invention
The present invention relates to dried semi-permeable membranes which are used especially as reverse osmosis membranes or as ultrafilters. More particularly, the dried semi-permeable membranes of the present invention are fabricated of polyacrylonitrile or acrylonitrile copolymers which can be stored under dry conditions.
2. Description of the Prior Art
The use of semipermeable membranes for the desalination of sea water, the desalination of brackish water, the treatment of industrial food industry wastes and the like by reverse osmosis or ultrafiltration is well documented. The selective permeability of the semi-permeable membranes is a very important property in the technology for the separation of solution components by the membranes. Especially desired are semi-permeable membranes which inhibit the permeation of a solute but readily permit the permeation of the liquid medium (e.g. water) in the separation process. Certain semi-permeable membranes have been disclosed such as the cellulose-acetate membranes disclosed in Japanese Patent Publication No. 2818/1967, the polyamide semi-permeable membranes disclosed in Japanese Patent Publication No. 21885/1971, and the polyacrylonitrile semi-permeable membranes disclosed in Japanese Patent Publication No. 4625/1972.
From the viewpoint of use of the semi-permeable membranes it is preferable that the membranes have high mechanical strength so that they may be easily used. However, the above-mentioned conventional semi-permeable membranes must be stored under wet conditions and cannot be used if the membranes are allowed to dry even once because the characteristics of the membranes are substantially and irreversibly changed. Accordingly, when a reverse osmosis apparatus or an ultrafiltration apparatus is assembled with one of the conventional semi-permeable membranes, the semi-permeable membrane should be maintained under wet conditions without failure. Because of this requirement, the apparatuses are disadvantaged by the difficulties of bonding the semi-permeable membrane and the apparatus which is a serious problem and by the special precautions required for the transportation and storage of the semi-permeable membrane. When a solution containing a certain solute is treated with a semi-permeable membrane, high water permeation velocities are necessary through the semi-permeable membrane and the permeation of the solute through the membrane must be prevented as much as possible in order to attain an effective industrial process. In order to achieve high water permeation velocities, it is preferable to decrease the thickness of the semi-permeable membrane. Conversely, in order to maintain high mechanical strength of the membrane, the thickness of the semi-permeable membrane is preferably increased. An asymmetrical semi-permeable membrane having a thin, dense surface layer and an inner layer of an open-celled structure, which has low resistance to water-permeation, has both of the desired properties of high mechanical strength and high water permeability. If the thin, dense surface layer is complete in the asymmetrical semi-permeable membrane, it can be used as a reverse osmosis membrane which can remove low molecular weight solutes. On the other hand, if the thin, dense surface layer is incomplete and has many fine pores, it can be used as an ultrafiltration semi-permeable membrane.
In order to increase the rate of water permeation and to increase the mechanical strength of the membrane, most portions of the semi-permeable membrane should have a porous structure. In an open-celled structure, the pores should communicate, but no cellular voids should be present in the structure so that the proper mechanism for water permeation is operative. Water is kept within the porous portions (open-celled structure) of the porous semi-permeable membranes. If water is removed from the porous portions, the semi-permeable membranes usually deform and their original structure cannot be recovered even though the dried membranes are immersed in water. The well known cellulose-acetate semi-permeable membranes have this irreversible property. Polyacrylonitrile semi-permeable membranes which are hydrophobic compared to the cellulose-acetate membranes also have the same irreversible property. Japanese Patent Application Publication No. 6257/1972 shows that semi-permeable membranes should be kept under moist conditions or maintained by impregnating the membranes with a hydroscopic plasticizer in order to store the semi-permeable membrane.
A need, therefore, continues to exist for semi-permeable membranes which do not have to be stored under moist conditions.